What does "Chacun des deux fait la moitié du chemin." mean?
Is it some sort of idiom or what?
Nope, no idiom, just a standard sentence meaning "each of the two does half of the way", that can be two persons starting from different points and meeting at the middle of the distance, or an image to mean that each of the two persons makes similar concessions to reach an agreement.
It's similar to saying "it takes two" or something like that - it says "each of the two makes half of the road".